This invention relates to glues of the class useful in the manufacture of plywood. It pertains particularly to glues which are foamable to produce stable, high density foams.
The use of foamed glues in the manufacture of plywood is advantageous for many reasons, including, particularly, uniformity of plywood product and economy of glue use. Their application is illustrated in a number of U.S. patents, of which U.S. Pat. Nos. 2,323,831 and 3,905,921 are illustrative.
It is characteristic of the prior art foamed plywood glues that they are of low density, i.e. a density of less than 0.5 grams/cc., usually less than 0.3 grams/cc.. The low density inherently results from the foaming operation by which the glues are produced.
The prior art foamed glues are difficult to convey because of their high viscosity and low density. This in turn makes them unsuitable for application in conventional plywood glue application equipment such as spreaders, curtain coaters, and sprayers. Specialized equipment, accordingly, is required to convey the glues and to spread them in the uniform spreads required to achieve a manufactured plywood product of uniform properties.
It is the general object of this invention to provide thermosetting resin glues which are foamable to high density, i.e. a density above 0.5 grams/cc., and to viscosities in the broad range of from 1,000 to 20,000 centipoises.
Further objects of the present invention are the provision of thermosetting resin glues which are foamable to produce adhesive foams which are stable in density and viscosity, which can be readily conveyed, and which can be applied without difficulty by the spreaders, curtain coaters and spray applicators which are standard equipment in the conventional plywood mill.
Still another object of the present invention is the provision of foamable glues having the above mentioned properties which in addition are highly adhesive, having bonding properties superior to conventional glues on an equal weight basis, or equal thereto when used at lower spreads of foamed glue.
Broadly considered, the foamable glues of our invention comprise at least one aqueous alkaline thermosetting resin of the group consisting of the phenol-formaldehyde-, resorcinol-formaldehyde-, resorcinol-phenol-formaldehyde-, urea-formaldehyde-, and urea-melamine-formaldehyde-thermosetting resins, and at least one surfactant of the class consisting of:
(1) the fatty acid amides of the alkanol amines having the following structure ##STR1## is a fatty acid radical of 8 to 18 carbon atoms, and R.sub.2 and R.sub.3 are lower alkyl radicals, and
(2) the tertiary amine oxides having the following structure ##STR2## wherein R.sub.1 is an alkyl radical of 8 to 18 carbon atoms and R.sub.2 and R.sub.3 are lower alkyl radicals. The surfactant is used in amount sufficient to render the glue foamable with a non-reactive gas to a stable foam having a density of more than 0.5 and a Brookfield viscosity of from 1,000 to 20,000 centipoises at 25.degree. C. and 20 rpm.
Further included in the glues in amount appropriate to develop the desired properties are clay, a bodying agent, an alkaline material and water.